Add base::get to support rvalue-reference tuple

std::get in libstdc++4.6 doesn't have a overload for rvalue-reference
tuple. That implies std::get<0>(std::make_tuple(1)) is `const int&`,
instead of `int&&`.
This CL add base::get with correct type.

Review-Url: https://codereview.chromium.org/2047013002
Cr-Commit-Position: refs/heads/master@{#401758}
3 files changed